This invention is related generally to biodegradable plastic materials comprising natural starch and synthetic polymers. In particular, this invention is directed to processes for the preparation of such materials having improved biodegradability and physical performance properties.
The past decade has witnessed a renewed interest in the environment. The special attention afforded so-called "green" products and methods of production attest to the fact that commercial viability is not always contrary to resource conservation. Often, environmental regulations and/or restrictions provided the impetus for innovative methods, compositions and apparatus.
A case in point is the recent legislative trend toward banning landfill disposal of various waste materials. Many hazardous chemical and biological wastes have long been restricted, but as available landfill space becomes increasingly scarce, many plastic products and packaging materials will soon require alternate means of disposal and/or compositional changes to enhance the rate of biodegradation. Plastics currently form a significant fraction of the materials in landfills. According to some estimates, even though they comprise only 7% to 9% by weight, they make up 15% to 20% of the waste volume, and total more than 30% of the waste area, because the fraction is comprised mainly of thin films, sheets and the like. As the use of landfills becomes increasingly restrictive, the need to address plastic wastes will become even more pertinent. With decreasing petroleum reserves and rising costs of the plastics derived therefrom, the need for viable substitutes from alternative raw materials will increase.
Early concern over such issues led technical communities and government to recognize the need for appropriate use of biodegradable plastic materials. Starch is an abundant, low-cost, biodegradable polymer. It has long been contemplated that its use in plastic and/or film production would reduce the demand for petroleum-derived polymers and simultaneously relieve the adverse environmental impact caused by land-fill disposal. It is well-known that starch, alone, forms a brittle film which is sensitive to water and, therefore, is biodegradable, but it must be combined with other materials, typically synthetic polymers, in order to provide the physical properties necessary for practical, commercial applications.
By the late 1980's biodegradable plastics were applied (and, unfortunately, often misapplied) in various use situations. The misapplication of inappropriate or incompletely developed technology led to products which often did not meet performance claims and expectations. The so-called "first generation technologies" often lacked one or more of the following: (a) rate and/or extent of biodegradation, primarily due to limitations of starch incorporation; (b) necessary physical properties and related characteristics; (c) an economical means to effectively and efficiently manufacture starch-based blends; (d) intermediate product compatibility with conventional plastics product conversion processes; and (e) lower limits on film thickness caused by the use of non-gelatinized starch materials.
The search for degradable plastic materials meeting the requirements stated above has been an ongoing concern in the art. Additional research and development has led to new discoveries in the field of biodegradable plastics. Newer "second generation" technologies more closely provide the required and desired levels of biodegradability, physical performance and film thickness through acceptable manufacturing processes at reasonable product cost.
The pursuit and development of second and even third generation technologies has taken many approaches. Biodegradable components utilized include polylactic acids, polycaprolactones and polyhydroxybutyrate-valerates. However, starch continues to be the predominant material investigated in the pursuit of these goals. According to a recent industry overview of biodegradable plastic technologies, starch is the most common among the rapidly biodegradable components used in such polymers. Starch biodegrades in 1 to 2 weeks through composting, sewage, or anaerobic biodigestion. Further, because starch is a renewable resource, commonly produced and processed around the world, it has a low cost relative to other such components, especially those which are petroleum-based.
U.S. Pat. No. 4,076,117 discloses modifying the starch component, without gelatinization, as a filler and to improve mixing with synthetic polymers such as polyethylene. This granule-based technology is limited by the amount of starch which can be incorporated into the plastic composition, without detrimentally affecting the physical properties of the end product. Low starch content, however, decreases the rate and extent of biodegradation. Furthermore, non-gelatinized corn starch limits the thickness of any resultant film to approximately 1.0 mil. Rice starch granules are considerably smaller, but available only at a significantly greater cost.
U.S. Pat. No. 4,337,181 discloses that the starch component may be gelatinized to disrupt the granules, access the molecular structure, and achieve a uniform dispersion. After the starch is partially gelatinized, the gel is mixed with ethylene acrylic acid copolymer to produce a starch-polymer composite. A higher acrylic acid content facilitates aqueous dispersion. The result is enhanced physical properties and greater degradability due to higher starch content and improved molecular accessibility. However, such batch methods are time-consuming and often do not provide materials with the desired physical and performance properties. In addition, because the acrylic acid content, in large part, determines polymer cost, the high acid levels overall represent an increased cost and economic disadvantage.
As a result, the prior art is associated with a number of significant problems and deficiencies. Most are related to the non-homogeneous dispersion and incorporation of starch into the plastic composition, and result from the methods and processes currently used.
It is, therefore, an object of this invention to provide a starch-based polymer composition and method for preparing the same, overcoming the problems and shortcomings of the prior art.
It is another object of this invention to provide a method for the continuous preparation of starch-based polymer compositions.
It is another object of this invention to provide a method for the continuous preparation of starch-based polymer compositions which is quicker, more efficient than batch and/or semi-continuous methods, and which simultaneously provides compositions having excellent physical properties and performance characteristics.
It is another object of this invention to provide a method for the continuous preparation of starch-based polymer compositions comparable with a wide variety of auxiliary processing techniques, including, but not limited to casting, simple extruding, milling, extrusion blowing and injection molding, as well as any other procedure which would lend itself to the formation of thin films, hollow tubing or rods, and the like, which could be used to manufacture biodegradable, finished products.
It is an object of this invention to provide a method for the preparation of starch-based polymer compositions such that copolymer components having significantly lower levels of acrylic acid, than otherwise thought possible, may be used, thereby achieving cost and related economic benefits.
It is yet another object of this invention to provide a method for the continuous preparation of starch-based polymer compositions such that the rate and/or extend of biodegradation is improved and/or independent of the starch content.
It is a further object of this invention to provide a method for the continuous preparation of starch-based polymer compositions such that starch gelatinization is optimized, thereby resulting in improved physical properties and enhanced biodegradability.
It is yet another object of this invention to provide a method for the continuous preparation of starch-based polymer compositions having improved composting characteristics.
It is an object of this invention to provide an improved starch-based polymer composition and a method of preparation through the addition of novel additives which enhance processing and result in excellent physical properties and performance characteristics.
It is an object of this invention to provide an improved starch-based polymer composition, one more efficiently prepared and simultaneously exhibiting excellent physical properties and performance characteristics through use of a low acrylic acid content copolymer.
It is a further object of this invention to provide a method for the continuous preparation of starch-based polymer compositions through pH control at an early process stage, such that starch gelatinization and starch-acrylic acid complexation are enhanced.
It is an object of this invention to provide a method for the continuous preparation of starch-based polymer compositions which permits control over water content, such that sufficient moisture is retained to lubricate the starch fraction without process complications.
These and other important objects will be apparent from the descriptions of this invention which follow hereafter.